Studies on the chemical, physical and mechanical properties of high-solidsclearcoats prepared from hydroxyl-terminated isophthalate-based oligoesters and a melamine resin

A series of high-solids (solvent-free) clearcoats was prepared from hydroxy l-terminated isophthalate-based oligoesters and the hexakis(methoxymethylol )melamine resin (HMMM). The oligoesters, which were used as binders, were p repared via a transesterification reaction of dimethyl isophthalate (DI) un der various molar ratios R (= mol ratio diols/diester) with a mixture of 1, 6-hexanediol (HD) and 2-methyl-1,3-propanediol (MP). Mixtures of the two di ols of different composition R, (= mol ratio HD/MP), were used as well. An attempt was made to investigate and correlate the chemical, physical and me chanical properties of the formulated solvent-free clearcoats to: (a) the a mount of the cross-linker (melamine resin); (b) the curing conditions (time and temperature); (c) the synthetic parameters applied in the preparation (via transesterification) of the oligoesters (e.g. excess of the diols in r eaction with the DI and the molar ratio between the two diols, the species of the Lewis acid catalyst). The properties of all the new formulated high- solids clearcoats were compared to those of a reference clearcoat and their cross-linking was correlated to their chemical structure and composition a s well as to their T, values. It was found that the R and R, values applied in the synthesis of the oligoesters (binders) significantly affected the p erformance of their corresponding clearcoats. More specifically, the clearc oats obtained from binders where 3 <R < 4, combined an enhanced processabil ity, higher values of non-volatiles (similar to 80%) (lower VOC content) an d better or comparable chemical, physical and mechanical properties (e.g. i mpact resistance, gasoline resistance, mar resistance, flexibility and penc il hardness) upon comparison to those of the reference clearcoat. (C) 2001 Elsevier Science B.V. All rights reserved.